In general, an optical module refers to different types of optical communication functions and/or devices accommodated in one package and modularized to be connectable to an optical fiber. Nowadays, bidirectional optical modules are mainly used, each formed by modularizing into a single module an optical transmitter using a laser diode light source with low power consumption and utilizable over long distance and an optical receiver which performs optical communication using a photo diode.
A bidirectional optical module includes an optical transmitter, an optical receiver, an optical filter, an optical fiber, and a holder for accommodating the optical fiber, etc. A transmission signal output from the optical transmitter passes through the optical filter and is incident on the optical fiber, and a reception signal output from the optical fiber is reflected by the optical filter and incident on the optical receiver.
The optical filter generally is tilted at 45 degrees and divides the transmission signal and the reception signal. However, when wavelength separation between the transmission and reception signals is several nanometer (nm) wide, it is difficult to separate the wavelengths using a difference between transmittance properties of S-polarized light and P-polarized light to have the optical isolation of the transmission and reception wavelengths (a difference between transmittance properties of the transmission signal and the reception signal) reach about 25 dB.
Accordingly, in a bidirectional optical module with separation between transmission and reception wavelengths of several nm, a beam splitter which distributes light power is used, not a wavelength division multiplexing (WDM) filter capable of dividing a wavelength. However, since the beam splitter transmits about 50% of incident light and reflects the other 50% of the incident light, many problems occur including problems described below.
First, to perform long distance transmission using a bidirectional optical module, high light coupling efficiency is necessary when an output from a laser diode is focused on an optical fiber. However, when a beam splitter is used, since 50% transmission and 50% reflection generate a filter loss of 3 dB, it is impossible to obtain an adequate light coupling efficiency of a laser diode.
Second, to perform the long distance transmission using the bidirectional optical module, in addition to the high light coupling efficiency of the laser diode, also the high reception sensitivity of a photo diode is necessary. However, it is impossible to obtain a high reception sensitivity due to the filter loss of 3 dB because of light output from an optical fiber being reflected by a filter to become incident on the photo diode.
Lastly, since adequate light isolation of transmission and reception signals is not provided in a bidirectional optical module with a wavelength separation of several nm, the transmission signal may have an effect on the reception signal and thereby deteriorate communication quality, e.g., optical crosstalk. Thus, there is a need for further improved technology relating to the long distance transmission using a bidirectional optical module.